Why is it so hard to make a hub with 2 or 3 Full speed USB-C ports for external drives?
Because (without DisplayPort 1.4
and displays that use it) you can't drive a 4k@60Hz display Alt Mode
and a USB 3 device (at 5Gbps) off a single USB-socket without going to the much more expensive Thunderbolt or USB4 protocols.
What is not so obvious is why almost every "USB-C hub" on the market has to include a video output and why there's no alternative of the good old, cheap and cheerful, USB 3 type A hub with USB C connectors. I think the awkward truth there is that - once you count in the PC using market - the "standard" connector that the majority of people have on the ends of their peripherals is still USB-A.
Reality is that - unless you pay the premium for Thunderbolt or USB 4 devices - the "USB" you get over a USB-C connector is the same old USB 3.1 that you get over a USB-A connector, so there simply isn't an incentive to switch. Nothing much seems to have adopted the USB 3.2 standard (which allowed 2 USB 3 streams over a USB-C connection) and even 3.1gen2 is only really needed by the most expensive external SSDs.
If you really want multiple downstream "full speed" USB-C then there are USB4/TB4 hubs from Caldigit, OWC etc. that will do just that - at a price.